Radar system



C. E. NOBLES Oct. 18, 1960 United States Patent RADAR SYSTEM Charles E. Nobles, Baltimore, Md., assignor toWestinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Oct. 12, 1954, Ser. No. 461,726

6 Claims. (Cl. 343-17.1)

This invention relates to radar systems and more particularly to means for reducing the effect of ground clutter in a radar system.

While searching for air borne objects with a scanning radar system, difliculty is often encountered because a certain amount of the energy radiated from the radar antenna is reflected back to the antenna from the ground. In this respect the radar system detects too much, rather than too little, with the result that the picture on the radar scope is confused by unwanted echoes or ground clutter. Unwanted echoes arise by virtue of the fact that ground reflections, together with reflections from true air-borne objects, are detected by the radar receiver and tend to clutter the indication on the radar scope.

In other words, the unwanted ground reflections observed on the radar sodpe tend to obliterate the desired reflections from true air-borne objects.

Accordingly, it is an object of my invention to materially reduce the effects of undesirable ground clutter in a radar system. In accordance withmy invention,

hereinafter described, use is made of the fact that in scanning past a small air-borne object such as an airplane, the packet of returned echoes is smaller than the packet received from a large object such as the surface of the earth. In the preferred embodiment of the invention, the video output of the radar receiveris separated into two channels. The signals appearing in the first of these channels are applied to one of the grids of a tetrode amplifier tube with a polarity which is negative with respect to the cathode of the tube. 'The amplitude of ground reflections and air-borne object reflections in this negative channel is sufficient to cut oif elec tron flow in the tube. Signals in the other channel are delayed in time and applied to the other grid of the tetrode tube with a positive polarity. The amount of delay effected in the second channel is just greater than the amount of time required for the scanning beam of the radar set to be on a small air-borne object such as an airplane. With this arrangement, the positive signal returns from air-borne objects will reach the one grid of the tetrode only after the negative returns in the first channel have been impressed on the other gn'd. Hence,

the tetrode will conduct to indicate the existence of an the returns from the air-borne object will pass to the indicator scope of the radar system.

Further objects and features of my invention will become apparent from the following description taken in connection with the accompanying drawings whichform a part of the specification and in Which:

. 2,957,171- Patented Oct. 18, 1960 Figure 1a is a side view of the scanning beam of a radar antenna;

Fig. 1b is a top view of Fig. 1a; Fig. 2 is a plot showing the wave shape of a typical video signal produced by a radar receiver;

Fig. 3 is a block diagram of a radar receiver whic incorporates my invention;

.Fig. 4 is a schematic diagram 'my invention;

Fig. 5 is a plot of an example of the type of wave forms which might be impressed on the grids of the tetrode tube shown in Fig. 4; and

Fig. dis a plot of an ideal wave form which can be applied to the'indicator scope of the radar set by use of the present invention. Referring to Figs. 1a and lb, the general scheme of a radar scanning system is shown. A parabolic radar antenna 10 rotates about an axis 12. The radar beam 14 is conical in shape; and as it passes over various objects, reflections are sent back to the antenna 10 to be detected by the receiver of the radar set. In the illustration of Fig. l, reflections will be received from air-borne object '16 and also from the surface of the ground since the beam strikes the ground. at point 18. The reflections from the ground are those which clutter the indication on the radar scope, thereby making it difiicult to observe and detect the returns from object 16.

.It is to be understood that the circular scanning system shown'in Fig. 1 is'- illustrative of only one type of of the gating circuit of scanning system which may be used with the present invention. Other well known types of scanning systems which may be used equally well are, for example, conic'al'scanning and spiral scanning. Likewise, the radar antenna 1'0"need not be located on the ground but may be located on anair'craft if desired. 'Referring to Fig. 2, the Wave shape of the video output of the radar'receiver includes various types of pulse forms which are repeated at uniform intervals. The transmitted pulse repetition rate of the radar transmitter 'is indicated by thetime elapse As each pulse is transmitted, at certain amount of the RF energy will be deflected to the receiving portion of the radar antenna and detected by the receiver of the system. Hence, a series of pulses A1, A2, A3, etc. (called main bangs) will appearin the video wave shape. As the successive transmitted pulses are sent out from the antenna of the system, ground reflections. will be detected by the receiver and will appear as pulses B1, B2, B3,-etc. The ground return pulses will occur between each main bang since a certain amount of the transmitted energy of each pulse will be reflected from the ground during this time. When the radar beam scans past an airborne object, such as object 16 shown in Fig. l, a packet of reflected pulses will appear'in the video wave form, These pulses are indicated in Fig. 2 as C1, C2, and C3. Since only three reflected pulses from object 16 appear in the wave form, it is apparent that the time required to scan object 16 is three times the pulse repetition rate. That is, since there are only three echoes reflected from object 16, only three pulses were generated by the radar transmitter du'r'ing'the time the beam intercepted object 16 as it traveled its circular path as shown in Fig. 1. Also included in the Wave shape of Fig. 2 are small amplitudevariations 20 called noise. These small amplitude variations may be caused by any one of a number of factors which cause random electrical fluctuations in the received signal as, for example, thermal agitation.

In Figs. 3 and .4 it can be seen that the receiver of the radar system isn't the superheterodyne type in which the frequency of an incoming signal is heterodyned to a new radio frequency (called the intermediate frequency), then amplified and finally detected. Received. signals. from antenna10 are fed into a mixer circuit 22. In this circuit 'the incoming signals received from antennaI'O are mixed with the output of local oscillator 24 to produce the intermediate frequency referred to above. .The output? of mixer ci'rcuit22 is amplified by amplifier 26. .ahfdlth en detected by the detector, -28. The detected output is finally amplifiedby amplifier 3.010. ranc thejw'ave form' shown in Fig. 2. The output side of amplifier 30 leads to two channels '32 and 34. -'The"signals.'-in' Ehan1iel 32 are delayed by delay circuit 36 and thosefinclfannel 34 are inverted in phase by circuit- 38. Thesignals from -delay circuit 36 and circuit 38 are comparediirrgating .cirsuit 40 to produce a tracking .gate signal'vvhicli is free from ground clutter. i

Detailed operation of the trackinglngateicircuitry 'is shown in Fig. 4. The detected signals 'ap'pearin'g inlchannel '34 have a-positive polarity and are' impressed (an-grid '42 of vacuum tube 44 in circuit 3 8. Theioutp ut of tube 44 is an amplified signal having a negative) polarity. I The negative signal is impressed on, grid 46 f t et-ro de tube 48 with an amplitude suflicient to cut off time whenever apulse A, B or C appears in thesignal The signal in channel 32 is first delayed in circuit=36 Since this type of circuit (i.e., the delay circuit) is weliknownin the an, a detailed description of its operation is .not included herein. It will be suflicient to say that circuit'36 de'lays the signals in channel 32 by an amount of time whichis just greater than the time required for the radar beam 14 to scan a distant air-borne objectas illustrated i nfliig. 1a. After being delayed in circuit 36, the signals in channel 32 are amplified by amplifier 31 and applied -to ;a. s ec ond grid 50 of tube 48 with a positive polarity. Anrplifier 37 is included in the present systemsincethe delay :circuit 36 is characteristically lossy and therefore, tends to reduee "the amplitude of signals in -channel 32. p g

Operation of the gatingcircuit-can best be understood by reference to Fig. which illustrates'the-wave shapes applied to grids 46 and 50 of tube-48. Bean-be seen that each wave shape is substantially-identical vvith that shown in Fig. 3, the only difference being that the signals in channel 34 now have a-negative-polarity and the signals in channel 32 are delayed by the time elapse The signals in channel 34 are shown-having a greater amplitude than those in channel32. This relationship may be altered, depending upon the amplification effected-in the respective channels. 'Whenever-a pulsedn channel 34 is applied to grid 46, tube 48 is' cut oflE. Even though the main bangs (A1, A2, etc.) and the ground-returns ('Bl, B2, etc.) in channel 32 are delayed by time elapsery, they :are received for a'much longer time thantime lapse 'y and, therefore, they will be cancelled by other like signals in channel 34 which are not delayed. -This factor is illustrated in Fig. 5 wherein del-ayedtpulses A1, B1, A2, -32, etc. are cancelled by pulses A4, 'B4, A5, B5, etc.-which 'lead the delayed pulses by time elapse 7. .-However, the -returns from object 16 (C1, C2, 03) have "allbeen-impressed on grid 46 in' channel 34 bythetime the corresponding signals in channel 32 reach grid 50. Therefore, these signals, having a positive'polarity, will pass to the :plate circuit 52 of tube 48 and, hencegtothe indicating .scope of the radar set.

Ideally, the signal in plate circuit52 will appearas the 'wave form illustratedin Fig. '6. Actually,ithe wave form may include stray pulses not 'foundin Fig.- 6, Since the 'main bang and ground return pulses in the respective channels may not completely ca'nc'el. In any eve nt," the pulses C1, C2 and C3 will predominate =in'the output wave form and will clearly indicate rhe existence of an :airborne object without interference from ground clutter. If the radar beam should leavecontacfwith theground, three of the pulsed ground return signalsin delay channel 32 will be impressed on grid SO'after'the Flast ground return pulses in channel 34 are impressed-on 'grid 46. These three ground pulses will'pass to output circuit52 and will appear on theradar scope. However,;the ground return signals will be sufliciently translated in time with respect object returns will appear on the radar scope without interference from the three ground return signals which appear later in time.

Although my invention has been described in connection with a certain specific embodiment, it will be understood by those skilled inthe art-that various changes in form and arrangement of parts'm'a'y be made to suit requirements without departing from F the spirit and l scope of the invention.

I claim as my invention:

1. In combination with ascanning radar system employing a receiver which is adapted to 'detect'RF energy pulses which are=refiected froma' distant object, means for reducing the effects of ground clutter in said system and comprising the combination of a pair of channels into which the detected RF. pulses are fed, a device in one of said channel's forjdelayingj-the 'detctedfpulses appearing therein by anamount 'which 'is an integral multiple of the time elapse between successive transmitted pulses and which is slighjtly 'agreaterlthan the time required to scan a"'distant object, saidtime 'beui'g further determined by-the scanning rateand'the' distance of said-object, a circuitiforf producing-.- a .tracking,=g'ate in response to pulsed sig nals "received'ffr'om -said di'stafiit target, an lectron discharge .tfibe included ,in said cuit, an anode, cathode,;and a .pair ofgrids 'int'zlii ded in said tube, means for applying 'the delayedfpulses'in one channel to the first of said with a polarity which is positive withlrespect to the cathode of s'aid't'ub'e,

and means for applying the undelayed pulses '-in the other of said channels-tolthesecond of said gridswith a polarity which is negative respect to said cathode.

2. In combination with a scanning radar 'system ,em-

ploying a receiver which .is-adapted, to Idetef fr RF energy pulses which are reflected from a-distant swear-means for reducing the effects of ground clutter in said system and comprising apair of .channelslinto whichgthef'detected RFpulses are fed, a device in-oneof saidfchanfils for delaying the detectedpulses appearingthe'reinby an amount which is an-integ ral multiple of the time elapse between successive vtransmitted.p'ulsesand which is-substantially equal to the. time requiredfto .scan .a' distant object, said time being further determined Qby-the ning rateand the distance of said 6bject,-;and.an*electron discharge tube which conducts in. respdnseto' the dela'yed signals appearing in said .onechannl, .said tube ,being rendered non-conducting in response toi'the pulses appearing in the otherof said channels. v H l V H 3. -In a scanning -radar rsy'stem employing a receiver '-which is adapted to :detect .tar-getreturns ofcradiatediRl" energy pulses, the combination-of apairofchannelsdnto which detected RF pulses-are'-fed,;a deviceim-oneQof said channels for-"delaying signals appearing therein-by an amount of time substantially. equal to-;'the'-tirne required to scan adistant object, said time being an-eyen multiple of the time -.-elapse between suc;cessive tr-ansmitted pulses and-beingwfurther determinedzbythe scan ning rate of'thesystem and the rangeiof-a-'-target,,- an

- electron discharge 1 tube 1 having an anode and aacatho'de included therein, first sand second grids afor said time,

means for applying therdelayedwsignalsvinnsaid :one "channel to the first "of said'igrids 'rvvith a ipolarity WhlCh is positive with respect 3 to thCDdflthddG'i of said tube, :and

rnezinsfor applying the nndela'yed signalsrin-athei-other of said channels tothe: secondzof lsaidsgrids with amolarity which is negative 'witlifrespectrto' thecathodewof said tube whereby the tubewill-zbe 'rendered hon-conducting in response to signals appearingfin said other channeL,

4. In a scanning radar systenia including a'zreceiver for I energy pulses "which 'BICiICfiCtfiBd roma distantiob- 'jec't, a system' for reducing-the;.ifects of ground clutter in said receiver and comprising a'ipair of'ehannels nto ich received 'energyepulsesrare' fed, means in one of said channels for delaying the signals appearing therein by an amount of time substantially equal to thetime required to scan a distant object, said time being an even multiple of the time elapse between successive transmitted pulses and being further determined by the scanning rate of the system and the range of said object, and means connected to said channels and responsive to the signals appearing therein for producing an output signal only in response to energy pulses reflected from said distant object.

5. The combination claimed in claim 4, wherein the output signal producing means is rendered conducting in response to the delayed signals in said one channel and non-conducting in response to undelayed signals in said other channel.

6. Apparatus for separating continuously reoccurring pulsed signals in a train of signals from pulsed signals which appear in said train for a predetermined period of References Cited in the file of this patent UNITED STATES PATENTS Dickson Sept. 26; 1950 2,566,189 Gloess Aug. 28, 1951 2,659,079 Cunningham Nov. 10, 1953 2,659,080 Benfield Nov. 10, 1953 2,714,205 Grayson et al. .v July 26, 1955 

